Currently, selective call receivers (SCR's) that receive FSK signals utilize analog demodulation circuits. These circuits employ conventional front-end mixers for mixing a high frequency FSK signal (e.g., 900 MHz) to an intermediate frequency (IF) FSK signal (e.g., 455 KHz). Generally, the IF-FSK signal is demodulated into baseband analog signals by a discriminator circuit (that converts frequency signals to multi-level baseband voltage signals) and a post-detection filter. For a 4-level FSK signal, for example, the output of the discriminator is a four level voltage signal, each voltage level representing a symbol (2 bits) of data.
Thereafter, the output of the discriminator is conditioned by a conventional post-detection filter which removes high frequency noise. Finally, the conditioned signal is processed by a conventional symbol synchronizer and a conventional symbol decoder, which convert the 4-level signal into digital symbol data (i.e., two data bits of information).
A substantial portion of these circuits are designed with analog circuit techniques. Although these circuits are generally effective in demodulating FSK signals, they characteristically have several undesirable drawbacks. The most significant drawback being that of high energy consumption, which has an adverse effect on the battery life performance of the SCR's. Another significant drawback is the difficulty in manufacturing high quality SCR's that utilize these analog circuits.
A significant portion of defects in the manufacture of SCR's is found in the analog receivers contained therein. Because of the parametric variances encountered with analog circuit components, an RF tuning step is required for the analog receiver. It is this step where most manufacturing defects are found, and where the manufacturing throughput of SCR's is adversely affected. Yet another difficulty is encountered when analog receiver circuits are integrated into IC's. The disadvantage arises from the need of having to redesign the analog receiver circuit when an IC fabrication process is upgraded to a higher performance fabrication process, e.g., 1 micron to 0.5 micron technology.
In sum, although analog circuits are effective in the design and manufacture of SCR's, their use in SCR's has an adverse affect in manufacturing quality, and is costly when IC fabrication processes are upgraded.
Accordingly, what is needed is a method and apparatus that overcomes the deficiencies of prior art analog receivers. In particular, a method and apparatus is desired that would allow for demodulation and symbol synchronization of a multi-level signal without the drawbacks of prior art analog receivers discussed above. Moreover, an apparatus is desired that would be substantially immune to manufacturing defects, and would be readily portable across different IC fabrication technologies.